Brake disc for a disc braking system

ABSTRACT

A brake disk has a thickness D1 and includes two friction plates disposed parallel to each other, between which an airflow duct having a width D2 is located in a substantially central position. The brake disk is made of a gray cast iron material containing molybdenum. The ratio of the disk thickness D1 to the airflow duct width D2 is 3.0≦D1/D2≦5. The brake disk has a great resistance to temperature change and a long operating life.

BACKGROUND AND SUMMARY OF INVENTION

The invention relates to a brake disk for disk brakes, especially fordisk brakes of utility vehicles.

In EP 0 778 355 A1 such a brake disk is disclosed, which has a diskthickness D1 and two friction disks arranged parallel to one another,between which an air channel lying substantially centrally with a widthD2 is formed. The ratio between the disk thickness D1 and air channelthickness D2 is about 3.6 in a disk casting. The disk brake consists ofa gray cast iron material with a relatively low molybdenum content of0.01 to 0.6 percent.

It is already known also from WO96/07766 (PCT/SE95/00927) to provide thegray cast metal with an addition of molybdenum. However, it is stated inthis publication that a molybdenum content of 0.3% or more isdisadvantageous, since it makes the cost of the material increase toogreatly and manufacture is difficult.

A brake disk of gray cast iron material with a molybdenum content isalso disclosed in DE 40 26 611 C2.

The design of the generic brake disk has proven to be basically sound.In this connection, see DE 195 44 559 C1. There is a need forimprovement, however, with regard to a further reduction of the disk'stendency to form cracks and with regard to increasing the life of thedisk. The same problem is also true of the brake disk disclosed in DE 4026 611 C2. Additional state of the art which deals with brake disks andtheir materials is disclosed in DE 44 26 0911 A1, DE 37 04 679 A1, DE-OS22 35 627, DE 94 00 562 U1 and DE 87 08 264 U1.

Setting out from the generic state of the art the invention is addressedto the problem of optimizing the brake disk in regard to a furtherreduction of the tendency toward cracking and in regard to increasingthe useful life.

The invention solves this problem in the generic brake disk and in brakedisks in which the ratio between disk thickness D1 and air channel widthD2 satisfies the condition

    3.0≦D1/D2≦5,

that the brake disk consists of a gray cast iron material with amolybdenum content that amounts to 0.6 to 0.8 percent.

The invention thus again departs from the trend of the industry,according to which the width of the cooling channel is to be maximizedin order on the one hand to enlarge the cooling streams between thefriction rings and on the other hand to minimize the weight of the brakedisk. The invention makes use of the knowledge that possibly too muchattention has been paid to the parameter, "disk weight,"0 the past.Another feature of the brake disk of the invention is to be seen in thematerial, according to which a greater molybdenum content in the graycasting is between 0.6 and 0.8% again contributes to a decidedlengthening of the life of the disk. In the experiment, increases ofuseful life of more than 40 percent, and typically even 60 percent,compared with variants with no molybdenum content.

The invention also, by the selection of the cooling channel width,increases the heat capacity and the rigidity of the brake disk, on theone hand, and at the same time reduces the brake disk to a genuineminimum. Thus, in the experiment under extreme conditions, the usefullife until the disk cracks through is often even doubled and thus thelife of the disk is definitely lengthened. Nevertheless, the cooling isonly unimportantly poorer than in the case of the disks of the state ofthe art in which the ratio D1/D2 is around 2.5 to 2.8.

Especially preferred variants of the invention are distinguished by thefact that the ratio between disk thickness and air channel widthsatisfies the condition 3.0≦D1/D2≦5. At the same time the invention isespecially suitable for brake disks with a diameter that is greater thanor equal to 300 mm (e.g., 430 to 460 mm), the thickness D1 being about45 mm and the air channel width D2 being 12 mm. This brake disk designis suitable for example for utility vehicle disk brakes with 22.5"(571.5 mm) [wheel rims], and it is especially advantageous for utilityvehicle disk brakes with a ratio between the disk's inside diameter andthe disk's outside diameter of about 0.6.

According to the invention the brake disk is made from a gray ironcasting (iron, traces of other alloy metals and impurities caused byfusion) with the stated molybdenum content. Compared with conventionalbrake disks, the brake disk of the invention offers an increase of theuseful life by a factor of 3 to 4, without thereby appreciablyincreasing the brake pad's liability to wear. Preferably, the molybdenumcontent of the gray casting is between 0.75% and 0.76%, especially at0.752%. A special advantage of the high molybdenum content is that,being a microcarbide former, the molybdenum prevents the propagation ofheat cracks in the brake disk. In contrast to the known niobiummicrocarbide former, molybdenum also increases the thermostability ofthe gray casting. Especially in combination with the geometric design ofthe invention, there is such an increase internal combustion engine helife of the disk that it even pays to increase the Mo content stillfurther.

Advantageously, the gray cast iron also contains chromium (Cr) in anamount of 0.2 to 0.3%. Chromium forms the microcarbide in conjunctionwith the molybdenum. Preferentially the brake disk gray casting has atotal of the following additives: C: 3.8-4%, Si: 1.4-1.8%, Mn: 0.6-0.8%,P: <0.1%, S: K 0.12%, Cr: <0.2-0.3%, Cu: <0.2%, Ni: <0.1%, Mo: 0.6 to0.8% and V: <0.1%. dr

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is further described below in connection with the drawing,wherein:

FIG. 1 is a partially sectioned view of a brake disk according to theinvention, and

FIG. 2 is a test diagram of hot-crack tests.

DETAILED DESCRIPTION OF THE DRAWINGS

The brake disk 1 has two friction disks 3 and 4 joined together bybridges 2, so that an air and cooling channel 5 is formed between thefriction disks aligned parallel to one another. On the innercircumference of friction disk 3 a pot-like projection 6 is formed forthe mounting of the brake disk 1 on a wheel axle. The brake disk 1 canalso, of course, be made multipartite--e.g., in the manner of DE 195 44559 C1--and/or it can be discontinuous.

The brake disk 1 represented in FIG. 1 is in practice made such that thetotal disk thickness D1 is 45 mm and the air channel width D2, 12 mm. Inthe case of brake disks whose air channel has a constant width, thewidth D2 is defined by the closest point. There is thus a ratio of D1 toD2 of 3.75 (in case of a wide diameter of 430-460 mm for 22.5" rims). Inthe state of the art the air channel width of 17 to 18 mm has been theestablished practice. The mass of the brake disk increases according tothe cooling channel geometry (web size etc.) by only about 2-3 kg. Italso proves advantageous that for the same energy input the temperatureof the brake disk increases less than in the case of larger air channelwidth and thus less heat capacity.

Any further reduction of the cooling channel width D2 is uneconomicalfor production reasons, since the sand core for the formation of the airchannel when the brake disk is cast is no longer manageable. The claimedratio between the magnitudes D1 and D2 is rather an optimization of thebrake disk geometry according to the parameters: danger of cracking,weight, cooling performance, producibility and economy.

FIG. 2 shows by way of example, with the aid of test diagrams of commonstress and heat cracking tests, that in utility vehicle brake disks witha thickness D1 of 45 mm and a ratio between disk inside diameter anddisk outside diameter around 0.6, with a reduction of the air channelwidth to about 12 mm (and to a value of around 12 mm in the hatchedcorridor), even in the case of a disk without Mo a clear prolongation ofdisk life beyond the life of a corresponding disk with an air channelwidth D2 of 17 mm can be attained. In comparison with the life of the"17 mm air-channel disk" the life increases by more than 100% when theair channel width is reduced to 12 mm. In the case of a brake disk ofgray cast iron with a molybdenum content around 0.6-0.8%, especially0.75% (and the following additional additives: C: 3.8-4%, Si: 1.4-1.8%,Mn: 0.6 to 0.8%, P: <0.1%, S: <0.12%, Cr: <0.2%, Ni: <0.1% and V: <0.1%)the useful life clearly increases again, especially when the air channelwidth of the disk with Mo added is around 12 mm. The life of the brakepad is reduced but minimally or not appreciably by the narrowing of theair channel.

We claim:
 1. A brake disk for disk brakes, comprising:a disk having athickness D1 and comprising two friction disks arranged parallel to oneanother and an air channel having a width D2 essentially centrallysituated between the two friction disks; wherein a ratio between diskthickness D1 and air channel width D2 is 3.0≦D1/D2≦5, wherein the brakedisk comprises a gray cast iron material having a molybdenum content of0.6 to 0.8%.
 2. A brake disk according to claim 1, wherein said brakedisk has a diameter of at least 300 mm.
 3. A brake disk according toclaim 1, wherein said brake disk has a diameter of 430-460 mm.
 4. Abrake disk according to claim 1, wherein the thickness D1 is about 45mm.
 5. A brake disk according to claim 1, wherein the width of the airchannel is about 12 mm.
 6. A brake disk according to claim 1, wherein aratio between a disk inside diameter and a disk outside diameter isabout 0.6.
 7. A brake disk according to claim 1, wherein the twofriction disks are equally thick.
 8. A brake disk according to claim 1,wherein the two friction disks are joined together by bridges.
 9. Abrake disk according to claim 1, wherein the brake disk is made as adiscontinuous, multipartite brake disk.
 10. A brake disk according toclaim 1, wherein the Mo content of the gray cast iron is 0.75% to 0.76%.11. A brake disk according to claim 1, wherein the gray cast ironcontains chromium.
 12. A brake disk according to claim 11, wherein thechromium content of the gray cast iron is 0.2 to 0.3%.
 13. A brake diskaccording to claim 1, wherein the gray cast iron has the followingadditives:C: 3.8-4%, Si: 1.4-1.8%, Mn: 0.6-0.8%, P: less than 0.1%, S:less than 0.12% Cr: less than 0.2-0.3%, Cu: less than 0.2%, Ni: lessthan 0.1%, Mo: 0.6 to 0.8%, V: less than 0.1%.
 14. A brake diskaccording to claim 1, wherein the ratio between disk thickness D1 andair channel width D2 is 3.2≦D1/D2≦4.
 15. A utility vehicle disk brakecomprising a brake disk according to claim 1, wherein said utilityvehicle disk brake has 22.5 inch wheel rims.